Piezoelectrically actuated fuel injection valve

ABSTRACT

A valve for controlling liquids which for its actuation is provided with a liquid-filled coupling chamber, which is disposed between an actuator piston of a piezoelectric actuator and a piston that actuates a valve member. To compensate for liquid losses suffered by the coupling chamber, which is briefly at high in each work cycle, the pressure difference that exists during the return stroke of the actuator piston between the coupling chamber and the opposite sides of the actuator piston and of the that actuates the valve member that are remote from the coupling chamber is utilized to achieve refilling in valveless fashion along gaps. The valve is used for use in fuel injection systems for internal combustion engines of motor vehicles.

PRIOR ART

The invention relates to a valve for controlling liquids One such valveis known from European Patent Disclosure EP 0 477 700. There, theactuating piston of the valve member is disposed, tightly displaceably,in a smaller-diameter portion of a stepped bore, while conversely alarger-diameter piston which is moved by the piezoelectric actuator isdisposed in a larger-diameter portion of the stepped bore. A hydraulicchamber is defined between the two pistons, in such a way that wheneverthe larger piston is moved a certain distance by the actuator, theactuator piston of the valve member is moved by an increased distance,the increase being due to the step-up ratio of the cross-sectional areasof the stepped bore. The valve member, the actuator piston, thelarger-diameter piston and the piezoelectric actuator are located inline with one another along a common axis.

In such valves, there is the problem of compensating for changes inlength of the piezoelectric actuator, the valve, the enclosed pressurechamber liquid, or the valve housing by means of the hydraulic couplingchamber. Since, to open the valve, the piezoelectric actuator generatesa pressure in the pressure chamber, this pressure also leads to a lossof pressure chamber liquid. To prevent the coupling chamber from beingpumped dry, refilling is necessary. A device which is supposed to effectsuch refilling is indeed already known from the prior art defined at theoutset, but it has the disadvantage that a constantly open communicationin both of the possible flow directions between the coupling chamber anda closed supply container, the latter being equipped with a certainconstant volume, has a substantial influence on the operatingperformance of the piezoelectric actuator. In particular, athus-increased volume leads to a compressibility that lessens thetransfer rigidity of the hydraulic column formed by the couplingchamber. Yet the known device essentially contemplates leakage from thecoupling chamber, in order to compensate for tolerances in the workingstroke. To counteract the attendant increase in compressibility,provision is made for adding stabilizing material, which has acompressibility-reducing effect, to the liquid in the coupling chamber.This purpose is served for instance by rubber or metal elements that areadded to the liquid.

ADVANTAGES OF THE INVENTION

The valve of the invention has the advantage over the prior art that thecoupling chamber always remains adequately well filled, becausereplenishing coupling liquid can flow toward the coupling chamber fromthe adjoining low-pressure chambers in the periods between the workingstrokes of the piezoelectric actuator. Any change in length of theoverall device that may occur is thus corrected on an ongoing basis. Therefilling or replenishment of the coupling chamber is accomplishedwithout problems via the piston guides. This is true even if thepiezoelectric actuator, the valve, the enclosed pressure chamber liquid,or the housing should change its length, for instance from warming up,because such a change in length in the coupling chamber is compensatedfor by leakage. It is also advantageous that the device functionssecurely and reliably, is simple in design, and assures secure, reliablesealing.

In an advantageous refinement set forth herein, the filling is promotedby the volumetric increase in the return stroke of the actuator piston,along with the piezoelectric actuator and the resultant pressure drop.

This pressure drop is advantageously also reinforced according to claim4, by a spring that urges the actuator piston toward the piezoelectricactuator. The invention is substantially improved by the provision gapsof defined size, which are designed for their task of refilling thecoupling chamber. The dimensioning rule recited herein promotes thissizing very substantially.

The planning of the construction of the piston that actuates the valveand of the actuator piston can be done on this basis, which says thatonly part of the length of the pistons determines the criteria thatdefine the communication between the low-pressure chamber and thecoupling chamber, while a remaining part of the piston in each casefurnishes the length that is required to assure exact guidance of thepistons. This is improved still further in which only a short gap lengthnear the coupling chamber is provided for the pistons, and where theliquid can be brought, unthrottled, out of the low-pressure chamber toquite near the gap l_(w) via the pressure fluid conduit.

A substantial improvement in the refilling according to the invention isobtained by setting a certain pressure, which is raised above theambient pressure, in the low-pressure chambers. This increases thepressure drop toward the coupling chamber, which promotes the refillingof the coupling chamber; this pressure is furnished as recitedhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Several exemplary embodiments of the invention are shown in the drawingsand described in detail in the ensuing description. Shown are:

FIG. 1, a fuel injection valve in section;

FIG. 2, a first exemplary embodiment of a piston arrangement for acoupling chamber with liquid replenishment;

FIG. 3, another design of a piston;

FIG. 4, a modification of the piston design of FIG. 3;

FIG. 5, a further modification of a piston design of FIG. 3;

FIG. 6, a graph of the course of the refilling over time;

FIG. 7, a design with three pistons; and

FIG. 8, an injection system having the fuel injection valve of theinvention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The valve of the invention is used in a fuel injection valve, which isshown in its essential parts in section in FIG. 1. This injection valvehas a valve housing 1, in which a valve needle 3 is guided in alongitudinal bore 2; this valve needle can also be prestressed in theclosing direction by a closing spring in a known manner and not shownfurther here. On one end, the valve needle is provided with a conicalsealing face 4, which cooperates, on the tip 5 of the valve housing thatprotrudes into the combustion chamber, with a seat 6 from whichinjection ports lead away into the interior of the injection valve, inthis case connecting the annular chamber 7, filled with fuel underinjection pressure, with the combustion chamber so as to execute aninjection once the valve needle has lifted from its seat. The annularchamber communicates with a further pressure chamber 8, which is inconstant communication with a pressure line 10, by way of which fuel isdelivered at injection pressure to the fuel injection valve from ahigh-pressure fuel reservoir 9. This high fuel pressure also acts in thepressure chamber 8, specifically on a pressure shoulder 11 there, by wayof which the nozzle needle can be lifted from its valve seat in a knownmanner, under suitable conditions.

On its other end, the valve needle is guided in a cylinder bore 12,where with its face end 14 it encloses a control pressure chamber 15that communicates constantly, via a throttle connection 16, with anannular chamber 17, which like the pressure chamber 8 is always incommunication with the high-pressure fuel reservoir 9. A bore that has athrottle 19 leads axially away from the control pressure chamber 15 to avalve seat 20 of a control valve 21. Cooperating with the valve seat isa valve member 22 of the control valve, which in the lifted state of thevalve establishes communication between the control pressure chamber 15and a low-pressure chamber 18 that communicates constantly with a reliefchamber. A compression spring 24 that urges the valve member 22 in theclosing direction is disposed in the low-pressure chamber 18 and urgesthe valve member 22 onto the valve seat 20, so that in the normalposition of the control valve, this communication of the controlpressure chamber 15 is closed. Since the area of the end face of thevalve needle 3 in the region of the control pressure chamber is largerthan the area of the pressure shoulder 11, the same fuel pressure in thecontrol pressure chamber as prevails in the pressure chamber 8 now keepsthe valve needle 3 in the closed position. If the valve member 22 islifted from its seat, however, then the pressure in the control pressurechamber 15, which is decoupled via the throttle connection 16, isrelieved. With the closing force now absent or reduced, the valve needle3 opens quickly, optionally counter to the force of a closing spring,and on the other hand can be brought into the closing position as soonas the valve member returns to its closing position, because from thattime on, via the throttle connection 16, the original high fuel pressurethen rapidly builds up again in the control pressure chamber 15.

The control valve of the invention has a piston 25 for its actuation,which acts on the valve member 22 and is actuatable by a piezoelectricactuator 32 not shown in further detail. The piston 25 is tightly guidedin a guide bore 28 disposed in a housing portion 26 of the fuelinjection valve, and with its end face 29, as can be seen from FIG. 2,it defines a coupling chamber 30, which is closed off on its oppositewall by an actuator piston 31 of larger diameter, which is in a bore 65and is part of the piezoelectric actuator 32 and which in addition canbe coupled in force-locking fashion to the piezoelectric actuator 32 bya spring washer 57 disposed in the coupling chamber 30. The return ofthe actuator piston together with the piezoelectric actuator 32 can alsobe done in some other suitable way instead. Both pistons 25 and 31 areguided tightly in their bores. Because of the different piston faceareas of the two pistons 25 and 31, the coupling chamber 30 acts as astep-up chamber, because it steps up a structurally dictated shortstroke of the piezoelectric actuator piston 31 into a longer stroke ofthe piston 25 that actuates the control valve 21. Upon excitation of thepiezoelectric actuator 32, the piston 25 is displaced far enough thatthe valve member 22 lifts from its seat 20. The effect of this is arelief of the control pressure chamber 15, which in turn brings aboutthe opening of the valve needle 3.

In FIG. 2, the coupling chamber 30 and the two pistons 25 and 31 areshown separately from the valve housing 1. The low-pressure chamber 18is disposed in housing part 26 on the side of the piston 25, while alow-pressure chamber 33 is disposed on the side of the piston 31 remotefrom the coupling chamber 30. The cylinder bores for the pistons 25 and31 have gaps 35 and 36 of width s1 and S2, respectively, by way of whichthe low-pressure chambers 33 and 18 communicate with the couplingchamber 30. The length of the gap 35 is designated l₁ and that of thegap 36 is designated l₂; the diameter of the piston 31 is d₁ and that ofthe piston 25 is d₂.

For actuating the valve member 22, the piezoelectric actuator 32 isexcited, and consequently the actuator piston 31 is displaced. Thisleads to a pressure increase in the coupling chamber 30, which in turnresults in a displacement of the piston 25 together with the valvemember 22. Because of the different diameters of the pistons, the piston25 moves farther in this process than the actuator piston 31. Thepressure increase in the coupling chamber leads to leakage losses ofcoupling chamber liquid via the leakage gaps between the pistons 25 and31 and their guidance in the bores. However, the time periods withinwhich a high pressure prevails in the coupling chamber, in order toactuate the valve member, are short in comparison to the time periods,or load pauses, in between.

In order for the coupling chamber 30 not to be pumped dry over thecourse of time via the gaps 35 and 36, at a high pressure that ensues invalve operation, the invention makes it possible by means of rapidrefilling of the coupling chamber 30 in the load pauses and also atrelatively low pressures in the low-pressure chambers 18 and 33, so asto compensate for any liquid loss that has occurred. This is promoted bythe fact that the actuator piston moves back again along with thepiezoelectric actuator when it is not excited. This is advantageouslyreinforced if the actuator piston is urged toward the piezoelectricactuator by a restoring force, which is preferably furnished by thespring 57 that is supported in the coupling chamber 30.

For this refilling, the two pistons 25 and 31 and their guides must bedesigned geometrically in a special way, to attain optimal operabilityof the arrangement and repeated restoration of the fill volume of thecoupling chamber 30. The goal, as the characteristic leakage rate value,is a geometric ratio in accordance with the following equation:${\frac{n \cdot d \cdot s^{3}}{V_{0} \cdot 1} \geq 4},$

in which d is the mean piston diameter in mm,

s is the gap width in μm,

l is the sealing gap length in mm,

n is the number of sealing gaps or pistons, and

V₀ is the initial volume of the coupling chamber in mm³, or even better,a ratio: $\frac{n \cdot d \cdot s^{3}}{V_{0} \cdot 1} \geq 8.$

From such a ratio onward, the fastest possible refilling is achieved,without tolerances, especially in the gaps 35 and 36, having any majorinfluence on the duration of the refilling. From the above ratio, itfollows that the gaps and the piston diameter selected should tend to belarge, and that the initial volume and the sealing gap length selectedshould be small. This characteristic leakage rate value of ≧8 should notbe selected as being overly large, however, because otherwise theleakage rate becomes too high and the coupling function, that is, thehydraulic rigidity of the coupling chamber filling volume becomes lessand thus the stroke becomes shorter. To keep the rigidity of thecoupling chamber 30, which is required for switching the valve, as highas possible, the initial volume V₀ of the coupling chamber should be assmall as possible.

If, for reasons of guidance precision and the attendant gap geometrythat should be kept constant for the two pistons 25 and 31, the gaps 35and 36 are not selected to be overly large, and the piston lengths l₁and l₂ are not selected to be overly short, and nevertheless thecharacteristic value should be${\frac{n \cdot d \cdot s^{3}}{V_{0} \cdot 1} \geq 4},$

then designs of the kind shown in FIGS. 3, 4 and 5 can be used for thepistons 25 and 31; in these designs, the hydraulically effective sealinggap length is reduced, or in other words is limited to a short lengththat defines the above characteristic value.

In FIG. 3, a piston 37 is shown whose length 1 is interrupted twice byannular grooves 38 and 39, so that despite a short sealing gap length,guide elements that are far apart are obtained, which improves theguidance precision. The gap lengths located between the annular groove39 and 38, the low-pressure chamber 18 and 33 and the coupling chamber30 are shorter than the original total length of the piston. The resultis a geometric ratio for the characteristic leakage rate value inaccordance with the above formula, which is more favorable for thefilling while having very good guidance precision.

In the design of FIG. 4, a piston 40 has an annular groove 41, which isdisposed near the coupling chamber 30 and thus there defines a shorteffective gap length l_(w). This short gap length enters only into thevalue obtained by the above formula. The piston part following thiseffective gap length serves as a necessary guidance part but has noinfluence of the value resulting from the above formula. In this way,the favorable value for refilling in the load pauses can be attained ina simple and reliable way.

Finally, in FIG. 5 a piston 42 is shown which compared with the versionof FIG. 4 with the short sealing gap length for the piston 40, ismodified such that here, one or more lateral flat faces 44 lead away tothe end of the piston from the annular groove 43, which is equivalent tothe annular groove 41 of FIG. 4. In such a design, the very short gaplength l_(w), which meets the above requirement is attained, yet theguidance of the piston 42 is still over a relatively long length andthus is precise. The gap width defined by the annular groove 43 and thelateral flat faces 44 is hydraulically so large that it is inoperativefor a sealing function, and the piston part that is determined by itslength acts only as a piston guide but does not enter into the result ofthe characteristic leakage rate value. The flat face 44 can beconsidered a pressure fluid conduit, through which the annular groove 43is supplied with pressure fluid from the adjoining low-pressure chamber.This flat face may be realized in some other way, instead, however, suchas in the form of a bore or some other kind of conduit between theannular groove 43 and the low-pressure chamber.

FIG. 6 shows a graph which with the three curves 45, 46 and 47illustrates the variable duration of the refilling in proportion to theduration of application of the operating pressure in the couplingchamber and at various ambient pressures. A time ratio is plotted on theordinate, which is determined by the length of time required to refillthe coupling chamber to a certain pressure, such as 90% of the ambientpressure, and the values of the leakage rate variable that result fromthe above formula at different parameters and with two gaps, that is,with two pistons, i.e., the pistons 25 and 31, are plotted ascending theabscissa. It can be seen that with large gaps, i.e. as the valuesresulting from the above formula increase, the refilling proceeds fasterand in a more favorable way. Conversely, for characteristic leakage ratevalues <4, the lengths of time tend to infinity. An essential factorhere is also the pressure that prevails in the low-pressure chamber.With increasing pressure, faster refilling is obtained.

In FIG. 7, a design with three pistons is shown, that is, with theactuator piston 31 already described and with the coupling chamber 30.However, here a piston that actuates the control valve 21 is embodied asa stepped piston, which is provided with two pistons 49 and 50.Consequently there is a total of three gaps 135, 136, and 135 here, byway of which liquid can escape from the coupling chamber and by way ofwhich the coupling chamber 30 must be refilled. For this kind of designas well, the refilling according to the invention can be employed. Itcan also be employed for devices with more than three pistons.

In an injection system of the kind shown in simplified form in FIG. 8,one injection valve 51 per engine cylinder, as described above inconjunction with FIG. 1, is used. The injection valve 51 is connected onthe one hand to a high-pressure reservoir 53 via a supply line 52, andto a low-pressure container 55 (tank) via a return line 54. Theinjection system also includes a fuel pump 56, a high-pressure pump 57,an overflow valve 58, a pressure control valve 59, a pressure limiter60, a flow limiter 61, and an electronic control unit 62.

According to the invention, a pressure holding valve 63, which is set toa pressure of from 10 to 20 bar, is inserted into the return line 54that leads from the injection valve 51 to the tank 55. The return line54 must then be embodied in a suitably stable way. In the injectionvalve 51, the two low-pressure chambers 18 and 33 which are located onthe two sides, remote from the coupling chamber 30, of the actuatorpiston 31 and of the piston 25 that actuates the valve member 22 areconnected to the low pressure, as already described, and this lowpressure is now held to an elevated level, for instance of 10 to 20 bar,by the pressure holding valve 63.

This kind of provision then effects rapid refilling of the couplingchamber 30 via the gaps 35 and 36 (see FIG. 2) in accordance with theequation${Q = {\frac{\pi \cdot d \cdot s^{3}}{12 \cdot n \cdot l} \cdot \left( {{P0} - {Pkopp}} \right)}},$

in which Q is the flow rate, d is the piston diameter, s is the gapsize, n is the dynamic viscosity, and l is the leakage gap length.

The use of a restraining valve 63 is especially recommended whenever thepressure difference between the pressure in the coupling chamber 30,which has dropped to approximately 0 bar after the actuator stroke, andthe ambient pressure of 1 bar until the next injection event of theinternal combustion engine (25 ms, for instance, at an engine speed of4800 rpm) is not sufficient for refilling the coupling chamber 30. Withthe differential pressure increased to from 10 to 20 bar, it is certainthat the coupling chamber 30 can be refilled within the short length oftime available. An advantage here is that only a single pressure holdingvalve 63 per engine is needed.

The foregoing relates to a preferred exemplary embodiment of theinvention, it being understood that other variants and embodimentsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

What is claimed is:
 1. A valve for controlling liquids, comprising a piezoelectric actuator (32), a valve member (22) which is actuatable in an opening direction via a piston (25) by said piezoelectric actuator (32) counter to a force of a spring (24), said piston (25) includes a first face end (29) which closes off a hydraulic coupling chamber (30), said hydraulic coupling chamber (30) is defined on a second side by a second face end of an actuator piston (31) which has a larger diameter than a diameter of the piston (25) and is a part of said piezoelectric actuator (32), a working stroke of said piston (31) increases a pressure in the coupling chamber (30), the piston (25) is adjusted by said working stroke and the pressure in said coupling chamber (30) counter to the force of the compression spring (24), a low pressure chamber (33) is formed on an end of said piston (31) remote from the coupling chamber (30) and a low pressure chamber (18) is formed opposite a second face end of said piston (25) remote from said first end face (29), via the pressure chamber (18) a piston (15) actuates the valve member (22), respective low-pressure chambers (18 and 33) are provided in which oil leakage pressure prevails, a gap (36) is located between the outer circumference of the piston (25) and a guide bore (28) and a gap (35) is located between an outer circumference of the actuator piston (31) and a guiding bore (65) by which oil leakage is directed to low pressure chambers (18 and 33), said guiding bores (28 and 65) along the pistons (25) and (31) are dimensioned such that whenever there is no pressure increase in said coupling chamber (30), the coupling chamber (30) is refilled from the low-pressure chambers (18) and (33) via said gaps (28 and 65) to compensate for leakage losses via said gaps into the low-pressure chambers that occur during high pressure periods, and the periods that are between these occurrences of pressure increases are shorter than the periods during which the pressure increases occur.
 2. The valve according to claim 1, in which a leakage loss in the coupling chamber (30) is compensated for by an increase in a volume of a coupling chamber pressure drop, occurring as a result of a return stroke of the actuator piston (31), between the coupling chamber (30) and the low-pressure chambers (18) and (33).
 3. The valve according to claim 2, in which the actuator piston (31) is coupled by a restoring spring (66) to the piezoelectric actuator (32) for a return stroke.
 4. The valve according to the claim 3, in which the coupling chamber (30) is refilled via the gaps (35) and (36) along a defined length l₁ and l₂, respectively, of the gaps of the pistons (25) and (31), and the gaps are dimensioned such that refilling of the coupling chamber (30) is always made possible within the periods between the individual working strokes of the piezoelectric actuator (32).
 5. The valve according to claim 2, in which the coupling chamber (30) is refilled via the gaps (35) and (36) along a defined length l₁ and l₂, respectively, of the gaps of the pistons (25) and (31), and the gaps are dimensioned such that refilling of the coupling chamber (30) is always made possible within the periods between the individual working strokes of the piezoelectric actuator (32).
 6. The valve according to claim 5, in which for refilling the coupling chamber (30), in the periods during which there are no pressure increases, the following geometric ratio is adhered to for the length and the width of the gaps, referred to the largest volume occupied by the coupling chamber: ${\frac{n \cdot d \cdot s^{3}}{V_{0} \cdot 1} \geq 4},$

in which V₀ is the volume of the coupling chamber (30) in mm³, n is the number of gaps that lead away from the chamber (30), s is the width of the gap (35, 136) in μm, 1 is the length of the gap in mm, and d is the mean diameter of the pistons in mm.
 7. The valve according to claim 6, in which the piston (25) for actuating the valve member (22) and/or the actuator piston (31) is subdivided in a length of its guidance in the respective bore (28) and (65) by at least one annular groove (38, 39, 41, 43).
 8. The valve according to claim 7, in which between the coupling chamber (30) and the at least one annular groove (41, 43), a short gap length l_(w) is defined which meets the geometric ratio, and the parts of the piston located on a far side of the at least one annular groove (41, 43) are embodied as parts (40, 42) used for guidance.
 9. The valve according to claim 8, in which between the at least one annular groove (43) and a side of the piston (42) toward the low-pressure chamber (18, 34), a pressure fluid conduit (44) is provided, by which the annular groove is supplied, unthrottled, with pressure fluid.
 10. The valve according to claim 5, in which for refilling the coupling chamber (30), in the periods during which there are no pressure increases, the following geometric ratio is adhered to for the length and the width of the gaps, referred to the largest volume occupied by the coupling chamber: ${\frac{n \cdot d \cdot s^{3}}{V_{0} \cdot 1} \geq 4},$

in which V₀ is the volume of the coupling chamber (30) in mm³, n is the number of gaps that lead away from the chamber (30), s is the width of the gap (35, 136) in μm, 1 is the length of the gap in mm, and d is the mean diameter of the pistons in mm.
 11. The valve according to claim 10, in which the piston (25) for actuating the valve member (22) and/or the actuator piston (31) is subdivided in a length of its guidance in the respective bore (28) and (65) by at least one annular groove (38, 39, 41, 43).
 12. The valve according to claim 11, in which between the coupling chamber (30) and the at least one annular groove (41, 43), a short gap length l_(w) is defined which meets the geometric ratio, and the parts of the piston located on a far side of the at least one annular groove (41, 43) are embodied as parts (40, 42) used for guidance.
 13. The valve according to claim 12, in which between the at least one annular groove (43) and a side of the piston (42) toward the low-pressure chamber (18, 34), a pressure fluid conduit (44) is provided, by which the annular groove is supplied, unthrottled, with pressure fluid.
 14. The valve according to claim 2, in which the coupling chamber (30) is defined by a face end of the actuator piston (31) and by a plurality of pistons (49) and (50).
 15. The valve according to claim 2, in which the pressure in the low-pressure chambers is kept at a predetermined level that is raised compared to the ambient pressure.
 16. The valve according to claim 1, in which the coupling chamber (30) is defined by a face end of the actuator piston (31) and by a plurality of pistons (49) and (50).
 17. The valve according to claim 16, in which the pistons (49) and (50) are combined into one stepped piston (48).
 18. The valve according to claim 1, in which the pressure in the low-pressure chambers is kept at a predetermined level that is raised compared to the ambient pressure.
 19. A fuel injection system which comprises a valve as set forth in claim 18, a high-pressure pump (57), high-pressure reservoir (52), and low-pressure container (55), and a low-pressure side of said valve is connected to the low-pressure container (55) and communicates with the low-pressure chambers (18) and (33) of the valve, a pressure holding valve (63) which is set to a pressure of over 1 bar is inserted into a return line (54).
 20. A fuel injection system as set forth in claim 19, in which the operative pressure in the low-pressure chambers (18) and (33) is set to from 10 to 20 bar. 